Stay tuned for more (or less): Neural selectivity and perception of numerosity and other quantities

In our daily lives, we have to process information about all kinds of quantities such as the set size of a group of items (i.e., numerosity), time and size, among others. We have previously reported specialized neurons in our brain which respond selectively to or ‘prefer’ a specific number of items such as one, two or three. These neurons are organized topographically which means that these neurons are laid out in a shape that allows those most closely related to communicate and interact over the shortest possible distance in the brain. The general research goal of this thesis was to examine the properties of numerosity-tuned neural populations, and numerosity perception as part of a generalized quantity system by investigating the possibility of shared, tuned mechanisms between numerosity and time, and other quantities and sensory modalities. We first examined whether and how the selective response or preference of these numerosity selective neurons can change based on recent sensory experience. We used the method of adaptation and ultra-high-field 7 Tesla fMRI, where participants were repeatedly shown a low or high numerosity so to adapt the numerosity-selective neural populations. Sensory adaptation, makes the appearance of subsequently presented stimuli appear more different from the adapting stimulus than they actually are. This method is a powerful tool which allows us to make inferences about the existence of specialized neurons in the brain which respond selectively to the adapting stimuli. We show the dynamic nature of numerosity selective neural populations, where neural numerosity selectivity was altered systematically in all numerosity selective brain areas. Based on findings showing brain regions which process more than one type of quantity, such as numerosity and time, we proceeded to use cross-adaptation to numerosity and time to study whether neural populations selective for numerosity or time interact. We found an unbalanced interaction between numerosity and time where adaptation to time affected numerosity perception but not the other way around. After finding this interaction between neurons selective for processing numerosity and time, further experiments showed that the neural populations underlying the effect of adaptation to time on numerosity perception are partially distinct from those underlying the effect of the time of adaptation on numerosity perception. Collectively, these results highlight that there are partially overlapping neural mechanisms which are dedicated for processing both numerosity and time. We propose that neurons which are selective or are ‘tuned’ to different quantities such as number, time or size are fundamental to understanding quantity perception. We illustrate how the properties of quantity-tuned neurons can underlie various perceptual phenomena. We further show that quantity-tuned neurons are organized in distinct but overlapping neural networks. We suggest that this overlap in tuning provides the neural basis for perceptual interactions between different quantities